"This has huge implications for ecosystems in the region, and for aspects of the human environment. For instance, natural gas facilities in the region, as well as power lines, roads, railways and buildings are all built on permafrost and are vulnerable to thawing. Such a thaw could damage this infrastructure, with obvious economic implications."

The international team studied stalactites and stalagmites from caves located along the ‘permafrost frontier’, where ground begins to be permanently frozen in a layer tens to hundreds of metres thick.

And analysis from a particularly warm period - Marine Isotopic Stage 11 - that occurred around 400,000 years ago suggest that another 1.5°C of global warming of compared to the present is enough to cause substantial thawing of permafrost a long way north of its present-day limit.

The team used radiometric dating techniques to date the growth of the stalactites and stalagmites. Data from the Ledyanaya Lenskaya Cave near the town of Lensk – the coldest region - shows that the only period when stalactite growth took place occurred about 400,000 years ago, during a period with a global temperature 1.5°C higher than today.

Periods when the world was 0.5-1°C warmer than today did not see any stalactite growth in this northernmost cave, suggesting that around 1.5°C is the ‘tipping point’ at which the coldest permafrost regions begin to thaw.

"Although it wasn’t the main focus of our research our work also suggests that in a world 1.5°C warmer than today, warm enough to melt the coldest permafrost, adjoining regions would see significant changes, with Mongolia’s Gobi Desert becoming much wetter than it is today and, potentially, this extremely arid area coming to resemble the present-day Asian steppes," says Vaks.

Methane may represent a ticking time bomb for global warming. It's already been discovered bubbling up from the sea bed in the East Siberian Sea; and, as the most potent greenhouse gas, there's a vicious circle in action. The more methane is released, the more temperatures are likely to rise, triggering the release of yet more methane.